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Proopiomelanocortin, a Polypeptide Precursor with Multiple Functions

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Proopiomelanocortin, a Polypeptide Precursor with Multiple Functions European Journal of Endocrinology (2003) 149 79–90 ISSN 0804-4643 INVITED REVIEW Proopiomelanocortin, a polypeptide precursor with multiple functions: from physiology to pathological conditions M L Raffin-Sanson1,3, Y de Keyzer1 and X Bertagna1,2 1Institut Cochin, De´partement d’Endocrinologie, Universite´ Rene´ Descartes-Paris V, France, 2Service des Maladies Endocriniennes et Me´taboliques, Hoˆpital Cochin, Paris, France and 3Me´decine Interne 2, Endocrinologie, Hoˆpital Ambroise Pare´, Boulogne/S, France (Correspondence should be addressed to X Bertagna; Email: [email protected]) Abstract Proopiomelanocortin (POMC) is the polypeptide precursor of ACTH. First discovered in anterior pitu- itary corticotroph cells, it has more recently been revealed to have many other physiological aspects. The fine molecular mechanisms of ACTH biosynthesis show that ACTH is but one piece of a puzzle which contains many other peptides. Present in various tissues, among which are pituitary, hypo- thalamus, central nervous system and skin, POMC undergoes extensive post-translational processing. This processing is tissue-specific and generates, depending on the case, various sets of peptides involved in completely diverse biological functions. POMC expressed in corticotroph cells of the pitu- itary is necessary for adrenal function. Recent developments have shown that POMC-expressing neur- ons in the brain play a major role in the control of pain and energy homeostasis. Local production of POMC-derived peptides in skin may influence melanogenesis. A still unknown function in the placenta is likely. POMC has become a paradigmatic polypeptide precursor model illustrating the variable roles of a single gene and its various products in different localities. European Journal of Endocrinology 149 79–90 Discovering proopiomelanocortin cell line, which is derived from an irradiation-induced (POMC) mouse pituitary tumour and selectively produces and secretes large amounts of ACTH, proved to be an The era of precursors of polypeptide hormones really invaluable tool. The latter technique indeed established started in the late 1960s with the discovery and that the small peptides that were identified (the end- characterisation of two prototypes: proinsulin, the products of the precursor processing, i.e. ACTH) were precursor to insulin, by Steiner in Chicago (1), and not mere degradation products generated during the the lipotrophins (b- and gLPH), the precursors to extraction procedure, since they were not obtained b-melanocyte-stimulating hormone (bMSH) by Li after the cells had been submitted to only short pulses and Chre´tien in San Francisco (2). Clues that a of labelling. These studies therefore showed that the precursor to adrenocorticotrophin (ACTH) might HMW ACTH precursor ultimately generated a material also exist came from the work of Yalow & Berson that was physically and immunologically indistinguish- (3) who found a high molecular weight (HMW) able from ACTH (4). Later, the same biochemical immunoreactive (IR) ACTH material in the extracts studies showed that labelled amino acids incorporated of a human thymic tumour responsible for a case into the HMW ACTH precursor were immunoprecipi- of ectopic ACTH syndrome. This biologically inactive tated by anti-ACTH but also by anti-bLPH and anti- molecule released bioactive ACTH under mild diges- bendorphin (bend) antibodies (5). Thus the precursor tion by trypsin. From these premisses the search for to ACTH also seemed to contain LPH sequences as an ACTH precursor was launched. well, an explanation for the previous observations The major progress came from fine biosynthetic reporting strictly parallel variations of ACTH and studies using labelled amino acids incorporated into LPH plasma concentrations in a variety of pathological newly translated proteins combined with pulse-chase conditions (6). strategies. This approach was feasible in a cell line The final step was carried out by Numa’s group: which had a high rate of ACTH synthesis. The AtT-20 the cDNA of the mRNA coding for the precursor q 2003 Society of the European Journal of Endocrinology Online version via http://www.eje.org Downloaded from Bioscientifica.com at 10/01/2021 08:33:00AM via free access 80 M Raffin-Sanson and others EUROPEAN JOURNAL OF ENDOCRINOLOGY (2003) 149 to ACTH was obtained and cloned (7). The sequence (NT). Exon 3 (833 bp) codes for most of the translated revealed – for the first time – the structure of the mRNA, i.e. the C-terminal part of the NT, joining ACTH precursor: ACTH itself was right in the middle, peptide (JP), ACTH, and bLPH (8, 9). Comparisons flanked by new sequences on its N-terminal end and between species show a strong homology between the by LPH on its C-terminal end. Thus the puzzle was ulti- DNA sequences coding for the NT, ACTH, bMSH and mately assembled, and established the molecular links bend, whereas the sequences coding for the JP and between ACTH, the LPHs, bend and putative new gLPH have poor homologies (10). peptides. The NT includes two intramolecular disulphide bridges, stabilising a loop structure composed of amphi- pathic amino acids. This conformational motif appears POMC expression and maturation in like a molecular sorting signal for POMC towards the human beings regulated secretory pathway (11). The POMC gene (Fig. 1) POMC gene expression in different tissues There is a single POMC gene per haploid genome (Fig. 2) in man. It is located on chromosome 2p23. It com- prises 7665 bp and consists of three exons and two POMC gene expression in the pituitary In normal introns. human pituitary, the POMC gene is only expressed in Exon 1 (87 bp) only contains untranslated corticotroph cells. Most mammals also possess an inter- sequences. Exon 2 (152 bp) codes for the signal peptide mediate lobe with POMC-expressing melanotroph cells and the first amino acids of the N-terminal peptide but this lobe is vestigial in adult man. Figure 1 POMC gene structure and expression. www.eje.org Downloaded from Bioscientifica.com at 10/01/2021 08:33:00AM via free access EUROPEAN JOURNAL OF ENDOCRINOLOGY (2003) 149 Proopiomelanocortin: a multiple function polypeptide precursor 81 Figure 2 POMC gene expression in normal and tumoral tissues. In corticotroph cells, a mature POMC mRNA of 1072 transcript is found by Northern blot as well as the nucleotides (nt) is generated, after the splicing of the 1072 nt pituitary mRNA and sometimes a larger primary transcript and a poly A þ tail of ,200 nt is 1450 nt transcript. POMC mRNAs are produced at added. The 801 nt of the coding region are translated a low rate but the placental mass explains why this into a prePOMC molecule starting with a 26 amino expression can influence plasma concentration of acid signal peptide necessary for the translocation of POMC-derived peptides during pregnancy (see the nascent protein through the membrane of the further) (19, 20). rough endoplasmic reticulum (RER). The peptide More recently, POMC gene expression and the pro- signal is rapidly cleaved. The protein is then engaged duction of melanocyte-stimulating peptides have been into the secretory pathway: at that time, the 241 demonstrated in keratinocytes, melanocytes and amino acid POMC molecule is made of the peptides dermal microvascular endothelial cells, raising the NT, JP, ACTH and bLPH, and is ready for maturation possibility of an auto/paracrine role influencing skin (or processing). pigmentation. The baseline expression of the 1072 nt transcript is upregulated by ultraviolet radiation (21–23). POMC gene expression in non-pituitary tissues POMC mRNA has been reported by different groups in POMC processing (Fig. 3) many normal non-pituitary tissues, in animals and man. In most of these tissues, POMC gene expression is Once POMC has reached the lumen of the RER, it fol- quantitatively and qualitatively different from that in lows the intracellular traffic of secreted protein through the pituitary: the tissue concentration of POMC the Golgi apparatus and ultimately the secretory gran- mRNA is extremely low, and the generated mRNAs ules where the end-products of the processing are are essentially short, truncated, transcripts of stored before being secreted by exocytosis. During this ,800 nt. These transcripts result from heterogeneous traffic the POMC molecule undergoes a series of proteo- transcription initiation at the 50 end of exon 3 (12– lytic cleavages and chemical transformations, which 15). They are non-functional and cannot be efficiently altogether result in the maturation or processing of translated into POMC. the precursor, yielding the various biologically active In the brain, however, a well-defined population of POMC-derived peptides. neurons – in the arcuate nucleus – do express a POMC is a prototype polypeptide precursor which POMC mRNA that is identical to that in the pituitary contains eight pairs, and one quadruplet, of basic (16–18). In these neurons POMC is produced and amino acids, which are potential cleavage sites for serves as the precursor to cerebral bend, aMSH processing enzymes (Fig. 1). In a given tissue the and other peptides which have biological functions nature of the POMC end-products indicates which in the brain. Recently the MSH system has gained sites are cleaved in this particular tissue. Thus, in attention as its role in energy homeostasis emerged the corticotroph cells of the anterior pituitary, only (see below). four of these cleavage sites are used, which are all The placenta is another physiological site of of the Lys-Arg type. Indeed, the following six peptides POMC-derived peptide production. The short 800 nt are generated: NT, JP, ACTH, bLPH, and a small www.eje.org Downloaded from Bioscientifica.com at 10/01/2021 08:33:00AM via free access 82 M Raffin-Sanson and others EUROPEAN JOURNAL OF ENDOCRINOLOGY (2003) 149 Figure 3 POMC processing by PC1 and PC2. amount of gLPH and bend, since the last cleavage acetylation of bend and aMSH). They contribute to site is only partially used (24–26).
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